1. Field of the Invention
The present invention generally relates to an object detection system coupled to and displaceable with a closed-loop belt.
2. Description of the Related Art
Landmines, airport luggage conveyor belts and food processing plant conveyor belts all have a need for detection of metal objects in a fast and efficient manner. In addition to detection of metal, these metal detectors sometimes must classify or discriminate the type of metal from clutter objects. This is particularly important for landmine detection to reject the potentially high incidence of metal clutter in the environment.
As shown in FIG. 1, the operation of metal detectors is based upon the principles of electromagnetic induction. Usually, a metal detector includes one or more transmitting coils or transmitters 10 carrying a time varying electric current generating a corresponding time-varying magnetic field 12, which propagates towards a metallic target 14. This primary or incident field produces eddy currents in the metallic target 14 generating, in turn, a secondary magnetic field 16, which is directed oppositely to the primary field and received by an antenna receiving coil or receiver 18, where it induces a detectable electrical voltage.
Metal detectors are proximity sensors having a region of sensitivity that is directly related to the size of the transmitter and receiver coils. Basically, the metal detector is sensitive to metal only near the receiving coil and is characterized by a time constant necessary for the metal detector to integrate or process the detected signal so as to discriminate metal from the background. The time constant is relatively fast when a human is walking and searching for metal objects such as, for example, landmines. However, in a variety of applications associated with the metal detector, this time constant is not adequate, particularly, in the context of military applications or relatively fast moving transporting belts. In a number of references, the U.S. Army has stated that a vehicle equipped with a metal detector must travel up to 10 m/sec or, approximately, 20 miles per hour (MPH) during landmine detection, which is substantially higher than the speed of a walking person. Assuming, for example, that a typical metal detector has a 0.2 m long metal detection region and a time constant of 0.2 sec, a speed of advancement of the vehicle equipped with this detector would be S=0.2 m/0.2 sec=1 m/s, which is not nearly sufficient to meet the U.S. Army guidelines.
To increase the efficiency of the detection, numerous designs of the vehicle-mounted metal detectors have been implemented. For example, U.S. Pat. No. 6,026,135 discloses a linear array of metal detectors fixed to a vehicle along a line perpendicular to the direction of travel. Similarly, U.S. Pat. No. 5,892,360 discloses an array of metal detectors fixed to a vehicle. These references only increase the metal detection area of coverage capabilities across the direction of motion (sweep area) and do not increase the detection capability in direction of motion.
In addition to detection of metal targets, it is necessary that these targets be discriminated from surrounding debris or metal clutter. To accomplish it, the metal detector must dwell over a metal target longer than the time needed to detect such a target. This is necessary because the signal identifying the target must be on the order of at least 10 times the noise for proper discrimination algorithms to work effectively. Accordingly, extra time is needed to integrate or process the signal and reduce the noise. For example, the time required for detection of small plastic landmines may be as long as 0.5 to 23 seconds, depending of the type of metal detector, type of metal, type of soil and the depth of the metal target
Increasing the efficiency of detection of metal targets is not limited to military and humanitarian demining. A variety of industrial processes including, for example, food and chemical processing, utilize conveyors for transporting material that has to be separated from metal targets. Conventionally, as diagrammatically illustrated in FIG. 2A, a plurality of linearly arranged metal detectors 20 are fixed to a stationary support 22 to detect metal object(s) 24 transported on a moving conveyor 26. Displacing the metal object 24 past by an array of the operating metal detectors 20 causes each detector to generate a respective output voltage signal V as long as the metal object 24 is within the field-of-view (FOV) of the receiver or antenna of the metal detector 20. Thus, the length of time T during the output voltage signal can be generated is determined by the size of the antenna, which is the combination of a metal detector's transmitter and receiver, of each metal detector 20 and the speed of the conveyor belt 26. The response time of the metal detector is governed by many parameters including, among others, magnetic field strength of the transmitter, duty cycle and timing of the transmitter, sensitivity of the receiver and the response time of the electronics configured to detect small metal signals in background noise. The response time of the electronics is characterized by the necessary signal averaging time, which is particularly important since all metal detectors must perform some type of signal processing, especially for small targets that may have signals buried in electronic noise.
The important point is all detectors 20 have a fixed response time. Typical response times are less than 1 second. Accordingly, for example, as long as the speed of the conveyor belt moves the object past the metal detector within its response time, metal objects will be detected reliably. As the speed increases, the fixed response time negatively influences the detection sensitivity and reliability. If very small metal objects are to be reliably detected, the response time needs to be increased to allow, for example, more signal averaging to reject background noise. In other words, the field of view (FOV) of the metal detector 20 must match the response time and sensitivity requirements of the metal detector. However, a typical response time of the detector(s) 20, mounted stationary relative to the conveyor belt 26, is low by comparison to the speed of this belt. As a consequence, while the metal detection technology is well developed and highly sensitive, its relatively long response time controls and, thus, limits the rate at which the detection can be performed.
Also, the discrimination of the metal object(s) 24 in the system having stationary metal detectors 20 may not be efficient since, as discussed above, the metal detector 20 does not have sufficient “dwell time” near the object 24 under study to enable the electronics to classify this object. As can be seen from FIG. 2B, a diagram of the typical metal detector output voltage versus position (or time in the case of an object moving across the array from left to right), the spatial/time variability of the output complicates any signal processing scheme that is trying to average the output of the spatial varying signal compared to a stationary output with good dwell time over the target. Particularly, a dwell or exposure time td, during which the metal object to be detected is within the field of view (FOV) of a respective detector, is relatively short. However, increasing the dwell time of the fixed detector(s) 20 may be a difficult task to accomplish, as the position of the object may be unknown. Using multiple fixed metal detectors does not provide for the sensitivity enhancing aspects of ensemble signal averaging that is possible in a single metal detector processing scheme.
A need therefore exists in a system for the detection of metal objects configured to provide reliable detection and classification of even relatively small metal targets at a relatively high speed of detection.